Wireless radio access technologies continue to be improved to handle increased data volumes and larger numbers of subscribers. The 3GPP organization is developing 5th Generation (5G) wireless networks to handle peak data rates of the order of ˜10 Gbps (gigabits per second) while still satisfying ultra-low latency requirements in existence for certain 4G applications. 5G intends to utilize radio spectrum on the order of GHz or more in the millimeter-wave (mmWave) band; the main target at this point is sub-6 GHz bands. 5G is also to support multiple input-multiple output (MIMO), and particularly massive MIMO with large antenna arrays.
In the development of the 5G system there has been discussion of the overhead for obtaining accurate channel state information (CSI). In this regard the following documents from the 3GPP TSG-RAN WG1 meeting #86 held in Göteborg, Sweden on 22-26 Aug. 2016 are relevant:    Document R1-167462 by Ericsson entitled Unified CSI reporting framework; and    Document R1-167463 by Ericsson entitled Pooling of RS resources for CSI reporting.
Mobile terminals (user equipment or UE) generally measure some kind of reference signal (RS) for this purpose but there are wide classifications for the various mobile users for which different types of RSs are suitable. Radio channels often experience either a large coherence bandwidth or a large coherence time. Equivalently one can say radio channels are either frequency flat versus frequency selective or stable over time versus time variant. Typically fast moving UEs with high Doppler are in-cars or other vehicles moving on a highway and which include more frequency flat radio channels due to the lower number of close by radiofrequency (RF) reflectors. On the contrary frequency selective radio channels are typically the result of urban non-line-of-sight (NLOS) scenarios where there is a large number of buildings nearby that act as RF scatterers, and which inherently leads to speed limited cars (<50, 60 or possibly 80 kmh on the high side). This class includes nomadic (pedestrian) UEs moving at only a few kilometers per hour.
In the LTE (4G) radio access technology there are two types of RSs that are relevant here for downlink CSI estimation. For nomadic users there are so called channel state information or CSI RSs; these are transmitted sparsely in frequency and time (for example, only every 5 to about 180 ms—this value is configured semi-statically). Each physical resource block (PRB) in LTE uses 40 resource elements (REs) for the support of up to 8 antenna ports so that the overall overhead remains in the low percentage range with a relatively good interpolation in the frequency direction, for example one value per PRB of 180 kHz.
LTE supports high speed UEs (up to 250 km/h) using common reference signals (CRSs) that the eNBs transmit in every PRB. This continuously generates a very high reference signal overhead of 5 to 10%, even if a cell contains only static users (indoor cells for example). Further, in multi-cellular environments CRSs provide only wideband channel information because different cells are orthogonalized by different Zadoff Chu sequences being spread over the full frequency bandwidth.
Merely combining CSI-RSs and CRSs to account for the different types of UEs, as the LTE systems does for certain transmission modes, adds to the signaling overhead. 5G is an opportunity to re-think that approach and one of the current 5G development goals is for an inherently flexible channel-estimating reference signal design that combines low to moderate overhead with support of a mixture of users where some have a high time variance (represented by fast moving UEs on a highway or train) and others have a high frequency selectivity (represented by nomadic users in a NLOS urban macro scenario). As detailed particularly below these teachings address that goal in a particularly elegant way. Though the examples are in the context of 5G these teachings have broader applications beyond only that radio access technology.
These aspects and others are detailed further below with particularity.